CRUSHING AND GRINDING
ment and very heavy wheels have been used with success, but only at the finest stage which might be considered in the classification of " crushing.7' Material 1 to 2 in. in diameter may be fed to such machines, and a product varying from
FIG. 8.—Hunting-ton mill.
y± in. to 20 mesh can be obtained from them. Since the most useful field for the
Chilean mill is in granulation or grinding, it will not be further considered here.
Ball mills are very widely used for reducing coarse rock to a product which
varies with the ideas of the metallurgist installing the plant. Ball mills are in use
FIG. 9.—Tube mill.
to 10 ft. in diameter, and for crushing work are of short length, not over 5 or 6 ft. Rock is fed to these machines up to 3 in. in diameter, although such practice is seldom considered economical. The best field for ball-mill crushing is with feed at about IK in- or less with a product varying from y± in. to 20 or 30 mesh. In